1. Field of the Invention
The present invention is directed to an automatic x-ray exposure unit suitable for use in mammographic examinations, which adjust the radiation dose, as defined by the mAs product, based on the particular film/foil system being employed and based on the patient thickness.
2. Description of the Prior Art
Several factors enter into the degree of blackening (exposure) of x-ray film in the production of x-ray images. For most types of examinations, including mammographic examinations, an optimum balance must be achieved in order to provide a readable exposed image, with the blackening being sufficiently dark to identify the details of the image, but not so dark that background blackening obscures those details. It is therefore known to automatically control the radiation dose in an x-ray diagnostics installation in order to achieve the desired degree of exposure of the x-ray film. Such adjustment is generally not a continuous, analog-type of adjustment, but rather constitutes a switching between one or more constant dose rates. Preferably other exposure parameters, such as tube voltage and current and the pre-filtering are not altered, i.e., the adjustment takes place independently of those parameters. The adjustment is necessary in order to take into account differing patient thicknesses. A given film/foil system, for example, may produce optimum blackening with a particular dose (mAs product) for a patient of nominal thickness, but may produce less than optimum results for a patient exhibiting a greater thickness. It is known to obtain a correction value, for use in adjusting the dose, based on the thickness of the subject by means of a double detector. Such a double detector obtains a value known as a transparency measurement, and is composed of two measurement cells, with the cell which is more remote from the x-ray source "seeing" radiation pre-filtered with a brass filter. Given a known tube voltage and a known prefiltering, the quotient of the two signals from the respective cells is thus dependent on the thickness of the subject, and can be employed for correcting the nominal dose value which has been set.
Leaving the possibility that the sensitivity of the detector cells themselves may be slightly energy-dependent, the measurements obtained by such a double detector are dependent not only on the examination subject, but also on the particular cassette and foil which are being used.
A basic curve 1 is shown in FIG. 1 for the mAs product dependent on the subject thickness, given a constant radiation dose at a measuring detector for a kV value. As noted above, the blackening decreases with increasing subject thickness.
In order to obtain a constant blackening (exposure level) at the film which is, of course, disposed behind the subject in the direction of radiation propagation, the mAs product must be corrected in a suitable manner, i.e., a higher mAs product must be provided, by switching the dose, given an increasing subject thickness.
For this purpose, a corrected characteristic curve 2 of the type shown in FIG. 2 is obtained for commonly used, commercially available film/foil systems, the corrected curve 2 being dependent on the nature of the film/foil system itself, the tube voltage and the pre-filtering.
In FIG. 2, the line 3 represents the calculated or nominal mAs value, the line 4 represents the amount of correction, and the line 5 represents the corrected mAs value, read from the corrected curve 2.
Given a constant pre-filtering, therefore, a characteristic curve must be calculated dependent on the tube voltage and on the particular film/foil system being used. This characteristic curve can be mensurationally calculated for all standard film/foil systems, and it is known to store such calculated curves in an automatic exposure unit. When a given, known film/foil system is used, the particular stored curve for that system, already calculated, is simply called from the memory and is used to undertake any necessary corrections.
Such correction curves, however, are only stored for film/foil systems which have already been tested, and thus a specific film development made for a customer, i.e., a customized film/foil system, will not have a stored curve associated therewith.